1. Field of the Invention
The present invention relates to a system for digitally measuring a vehicle parameter, such as vehicle speed or engine rpm and displaying the measurement on an analog gauge. More particularly, the present invention relates to a method and system whereby a variety of analog gauges with a multiplicity of programmable sweep and full scale ranges can be calibrated easily for use in a wide range of vehicle applications.
2. Description of the Prior Art
Several prior art references utilize a microprocessor and software for developing signals that are used to drive an air-core gauge in a speedometer or tachometer system. While microprocessor or microcontroller driven systems may exhibit reduced manufacturing costs, the cost of software development and changes required to support a multiplicity of different vehicle designs, engine and transmission combinations, wheel sizes, etc. demands a large investment in software and careful control of the manufacturing process wherein the software is inserted into the microprocessor. An example of a multiple microcontroller system for calibrating and displaying engine rpm and wheel speed is disclosed by Moran et al in U.S. Pat. No. 4,939,659.
Another approach found in the prior art is disclosed by Congdon in U.S. Pat. No. 4,356,445 which utilizes discrete digital electronics for generating sine and cosine signals in sync with the input signal for driving an air-core meter movement.
One approach often used by systems employing the direct digital approach teaches the measurement of time-based events by counting the individual events for a fixed period of time determined by the full-scale frequency (either the speed or rpm) in the desired sweep range (deflection in degrees) of the meter. Both of these constants must be programmed into the discrete electronics of the system and are therefore difficult to change.
Unfortunately, the frequency range of the signal is usually incompatible with the desired response time of the instrumentation system and drivers. For example, with a typical speed sensor output of 2.22 Hz/mph and using a 270.degree. deflection gauge for registering a maximum speed of 85 mph, the period of time required to achieve a 1/2.degree. resolution accuracy is approximately 1.5 seconds. Under these assumptions the electronics only will update the data value approximately every 1.5 seconds, which is well beyond the 50 millisecond update period normally required to achieve the appearance of continuous movement in the gauge. While it may be possible to use sensors having a much higher frequency output per revolution, these sensors are much more expensive and are bandwidth limited when the rotational speed of the axle or the engine crankshaft approaches very high levels.
In contrast to these prior art references, the present measurement system uses digital signal electronics that provide greater accuracy, linearity, monotinicity and speed of response than do conventional analog gauges. The present invention incorporates user program functionality for sweep, range, sender interface (either an 8-pole or VRS for the speedometer or 4/6/8 cylinder set-ups for the tachometer), and pointer placement offset for end of manufacturing line calibration. This programmability allows for a generic speedometer/tachometer electronics board to be manufactured and programmed for the application at final assembly and test. This commonality of design greatly reduces parts inventory, board inventory, and manufacturing complexity. In the preferred embodiment, up to 1024 possible design combinations can be accommodated with a single speedometer/tachometer electronics board.